Answer:
a)15 N
b)12.6 N
Explanation:
Given that
Weight of block (wt)= 21 N
μs = 0.80 and μk = 0.60
We know that
Maximum value of static friction given as
Frs = μs m g = μs .wt
by putting the values
Frs= 0.8 x 21 = 16.8 N
Value of kinetic friction
Frk= μk m g = μk .wt
By putting the values
Frk= 0.6 x 21 = 12.6 N
a)
When T = 15 N
Static friction Frs= 16.8 N
Here the value of static friction is more than tension T .It means that block will not move and the value of friction force will be equal to the tension force.
Friction force = 15 N
b)
When T= 35 N
Here value of tension force is more than maximum value of static friction that is why block will move .We know that when body is in motion then kinetic friction will act on the body.so the value of friction force in this case will be 12.6 N
Friction force = 12.6 N
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30 + 6 = 36
36/12 = 3
So, it would take it 3 hours to go 12 kms downstream.
Note that we added 30 and 6 because it was going downstream. If it was going upstream, then we would have had to subtract.
The British physicist Joseph John (J. J.) Thomson (1856–1940) performed a series of experiments in 1897 designed to study the nature of electric discharge in a high-vacuum cathode-ray tube, an area being investigated by many scientists at the time. Thomson's model showed the atom as a positively charged ball of matter with negatively changed electrons floating freely around inside of it. This model showed the atom having no structure. There are also no protons and neutrons in this model. Thomson knew that the atom had positively and negatively charges particles in it he just didn't know how they were arranged. <span>Today's model gives us a much clearer picture of the atom. There is a positively charged center of the atom that is denser than the rest of it called the nucelus. This dense center is made up of positively charged protons and neutrally charged neutrons. Around the outside of the nucleus the electrons are organized on rings. These electrons are arranged in a certain pattern that is the same for all atoms.</span>
